1. Field of Invention
The present invention relates to an improved method of, and apparatus for, diagnosing and treating amblyopic conditions in the visual system of human patients, in a safe, effective and inexpensive manner.
2. Brief Description of the State of Knowledge in the Art in the Field of Invention
Currently, a great deal has been written about the vision-related problem referred to as “amblyopia” which afflicts millions of individuals annually is the major cause of blindness throughout the world. Reference can be made to the following publications for conventional methods of detection and treatment for this condition:
American Academy of Ophthalmology: Amblyopia. Basic and Clinical Science Course: Pediatric Ophthalmology and Strabismus 1997; 259–265.
Daw N W: Critical periods and amblyopia. Arch Ophthalmol 1998 Apr; 116(4): 502–5;
Flynn J T: Amblyopia: its treatment today and its portent for the future. Binocul Vis Strabismus Q 2000 Summer; 15(2): 109;
Flynn J T, Woodruff G, Thompson J R, et al: The therapy of amblyopia: an analysis comparing the results of amblyopia therapy utilizing two pooled data sets. Trans Am Ophthalmol Soc 1999; 97: 373–90; discussion 390–5;
Flynn J T: 17th annual Frank Costenbader Lecture. Amblyopia revisited. J Pediatr Ophthalmol Strabismus 1991 July–August; 28(4): 183–201;
Kirschen D G: Understanding Sensory Evaluation. In: Rosenbaum A L and Santiago A P. Clinical Strabismus Management: Principles and Practice. 1999: 22–35;
Kushner, B J: Amblyopia. In: Nelson L B, ed. Harley's Pediatric Ophthalmology. 1998: 125–39; and
von Noorden G K: Binocular Vision and Ocular Motility: Theory and Management. 1996: 216–54.
As taught by these medical references, amblyopia refers to a decrease of vision, either unilaterally or bilaterally, for which no cause can be found by physical examination of the eye. The term functional amblyopia often is used to describe amblyopia, which is potentially reversible by occlusion therapy. Organic amblyopia refers to irreversible amblyopia.
Most vision loss from amblyopia is preventable or reversible with the right kind of intervention. The recovery of vision depends on how mature the visual connections are, the length of deprivation, and at what age the therapy is begun. It is important to rule out any organic cause of decreased vision because many diseases may not be detectable on routine examination.
Although many types of amblyopia exist, it is believed that their basic Pathophysiological mechanisms are the same even though each factor may contribute different amounts to each specific type of amblyopia. In general, amblyopia is believed to result from disuse from inadequate foveal or peripheral retinal stimulation and/or abnormal binocular interaction that causes different visual input from the foveas.
Three critical periods of human visual acuity development have been determined. During these time periods, vision can be affected by the various mechanisms to cause or reverse amblyopia. These periods are as follows:
Development of visual acuity from the 20/200 range to 20/20, which occurs from birth to age 3–5 years;
Period of highest risk of deprivation amblyopia, from a few months to 7 or 8 years; and
The period during which recovery from amblyopia can be obtained, from the time of deprivation up to teenage or even, sometimes, adult years.
Whether different visual functions (e.g, contrast sensitivity, stereopsis) have different critical periods is reported as not known in the scientific literture. It is said that in the future, determination of these time frames may help modify treatment of amblyopia.
In the US, the prevalence of amblyopia is difficult to assess and varies in the literatures ranging from 1.0–3.5% in healthy children to 4.0–5.3% in children with ophthalmic problems. Most data show that about 2% of the general population has amblyopia.
Amblyopia was shown in the Visual Acuity Impairment Survey sponsored by the National Eye Institute (NEI) to be the leading cause of monocular vision loss in adults aged 20–70 years or older. Prevalence of amblyopia has not changed much over the years.
Amblyopia is an important socioeconomic problem. Studies have shown that it is the number one cause of monocular vision loss in adults. Furthermore, amblyopes have a higher risk of becoming blind because of potential loss to the sound eye from other causes.
Amblyopia occurs during the critical periods of visual development. An increased risk exists in those children who are developmentally delayed, were premature, and/or have a positive family history.
Diagnosis of amblyopia usually requires a 2-line difference of visual acuity between the eyes; however, this definition is somewhat arbitrary and a smaller difference is common.
A common characteristic of amblyopic eyes is difficulty in distinguishing optotypes that are close together. Visual acuity often is better when the patient is presented with single letters rather than a line of letters. Diagnosis is not an issue in children old enough to read or with use of the “tumbling E.”
If the child protests with covering of the sound eye, amblyopia can be diagnosed if it is dense.
Fixation preference may be assessed, especially when strabismus is present.
Induced tropia test may be performed by holding a 10-prism diopter before one eye in cases of an orthophoria or a microtropia.
In infants who cross-fixate, attention should be paid to when the fixation switch occurs; if it occurs near primary position, then visual acuity is equal in both eyes.
Caution should be used when obtaining Teller acuity in children, as grating acuity may be less reduced than Snellen acuity, especially in strabismic amblyopia.
Strabismic and anisometropic amblyopic eyes have marked losses of threshold contrast sensitivity, especially at higher spatial frequencies; this loss increases with the severity of amblyopia.
Patients with strabismic amblyopia may have better visual acuity or less of a decline of visual acuity when tested with neutral density filters compared to the normal eye. This was not found to be true in patients with anisometropic amblyopia or organic disease.
Amblyopia usually is associated with changes in binocular function or stereopsis.
Some amblyopes may consistently fixate with a nonfoveal area of the retina under monocular use of the amblyopic eye, the mechanism of which is unknown. This can be diagnosed by holding a fixation light in the midline in front of the patient and asking them to fixate on it while the normal eye is covered. The reflection of the light will not be centered.
Cycloplegic refraction must be performed on all patients, using retinoscopy to obtain an objective refraction. In most cases, the more hyperopic eye or the eye with more astigmatism will be the amblyopic eye. If this is not true, one needs to investigate further for ocular pathology.
A full eye examination should be performed to rule out ocular pathology.
While many causes of amblyopia exist, the most important are as follows: Anisometropia; Strabismus; Strabismic anisometropia; Visual deprivation and Organic abnormalities.
In the case of Anisometrop, inhibition of the fovea occurs to eliminate the abnormal binocular interaction caused by one defocused image and one focused image. This type of amblyopia is more common in patients with anisohypermetropia than anisomyopia. Small amounts of hyperopic anisometropia, such as 1–2 diopters, can induce amblyopia. In myopia, mild myopic anisometropia up to −3 diopters usually does not cause amblyopia. Hypermetropic anisometropia of 1.50 diopters or greater is a long-term risk factor for deterioration of visual acuity after occlusion therapy.
In the case of Strabismus, the patient favors fixation strongly with one eye and does not alternate fixation. This leads to inhibition of visual input to the retinocortical pathways. Incidence of amblyopia is greater in esotropic patients than in exotropic patients.
In the case of Strabismic anisometropia, patients have strabismus associated with anisometropia
In the case of visual deprivation, amblyopia results from disuse or understimulation of the retina. This condition may be unilateral or bilateral. Examples include cataract, corneal opacities, ptosis, and surgical lid closure.
In the case of organic abnormalities, structural abnormalities of retina or optic nerve may be present. Functional amblyopia may be superimposed on the organic visual loss.
If suspicion exists of an organic cause for decreased vision and the ocular examination is normal, then further investigations into retinal or optic nerve causes should be initiated. Studies to perform include imaging of the visual system through CT scan, MRI, and fluorescein angiography to assess the retina.
Although differences in the electrophysiologic responses of normal versus amblyopic eyes have been reported, these techniques remain investigational and the differences are controversial.
The conventional course of medical care for amblyopic patients is as follows:
The clinician must first rule out an organic cause and treat any obstacle to vision (eg, cataract, occlusion of the eye from other etiologies).
Remove cataracts in the first 2 months of life, and aphakic correction must occur quickly.
Treatment of anisometropia and refractive errors must occur next.
The amblyopic eye must have the most accurate optical correction possible. This should occur prior to any occlusion therapy because vision may improve with spectacles alone.
Full cycloplegic refraction should be given to patients with accommodative esotropia and amblyopia. In other patients, a prescription less than the full plus measurement that was refracted may be prescribed given that the decrease in plus is symmetric between the two eyes. Because accommodative amplitude is believed to be decreased in amblyopic eyes, one needs to be cautious about cutting back too much on the amount of plus.
The next step is forcing the use of the amblyopic eye by occlusion therapy. Occlusion therapy has been the mainstay of treatment since the 18th century.
The following are general guidelines for occlusion therapy:
Patching may be full-time or part-time. In general, children need to be observed at intervals of 1 week per year of age, if undergoing full-time occlusion to avoid occlusion amblyopia in the sound eye.
Always consider lack of compliance in a child where visual acuity is not improving. Compliance is difficult to measure but is an important factor in determining the success of this therapy.
In addition to adhesive patches, opaque contact lenses, occluders mounted on spectacles, and adhesive tape on glasses have been used.
Establishing the fact that the vision of the better eye has been degraded sufficiently with the chosen therapy is important.
For children who will not wear a patch or in whom compliance is an issue,
penalization therapy can be used. The following are general guidelines for penalization therapy:
Atropine drops or ointment is instilled in the nonamblyopic eye. This therapy may be (and is preferably) used in conjunction with patching or occlusion of the glasses (eg, adhesive tape, nail polish).
This technique also may be used for maintenance therapy, which is useful especially in mild amblyopes.
Currently, the Amblyopia Treatment Study is ongoing to determine whether the success rate with atropine treatment of amblyopia as a primary treatment is equivalent to the success rate of occlusion therapy. The results of this study will be helpful in determining the exact role of atropine penalization in the treatment of amblyopia.
Other options include optical blurring through contact lenses or elevated bifocal segments.
The endpoint of therapy is spontaneous alternation of fixation or equal visual acuity in both eyes. When visual acuity is stable, patching may be decreased slowly, depending on the child's tendency for the amblyopia to recur. Because amblyopia recurs in a large number of patients, maintenance therapy or tapering of therapy should be strongly considered. This tapering is controversial, so individual physicians vary in their approaches.
Close supervision during occlusion therapy is necessary to make sure children do not peek. Various methods of preventing children from removing patches have been considered, from a reward system for older children to arm splints and mittens for infants.
Pharmacologic treatment with levodopa has been investigated and has showed transient improvement of vision in amblyopic eyes. However, the exact role of such pharmacologic agents has not been determined. Levodopa currently is not being used clinically.
Atropine penalization (with either ointment or drops) is an alternative method of blurring vision in the sound eye of patients who refuse patching. It may be applied once a day to patients in the preferred eye only.
Outpatient follow-up needs to continue beyond the primary completion of amblyopia treatment because visual deterioration occurs in many children. One study by Levartovsky et al showed deterioration in 75% of children with anisometropia of 1.75 diopters or more after occlusion therapy. Recidivism can occur even several years after the initial treatment period and is as high as 53% after 3 years.
The main complication of not treating amblyopia is long-term irreversible vision loss. Most cases of amblyopia are reversible if detected and treated early, so this vision loss is preventable. After 1 year, about 73% of patients show success after their first trial of occlusion therapy. Studies have shown that the number of patients who retain their level of visual acuity decreases over time to 53% after 3 years.
Risk factors for failure in amblyopia treatment include the following:
(1) the type of amblyopia, in which high anisometropes and patients with organic pathology have worse prognosis, and strabismic amblyopes have the best outcome.
(2) the age at which therapy began, in which younger patients seem to do better; and
(3) the depth of amblyopia at the start of therapy, in which the better the initial visual acuity in the amblyopic eye, the better the prognosis.
While the current state of medical knowledge and practice recommends a great deal in the way of treatment based primarily on occlusion therapy, such methods and associated apparatus offer little, if any, hope of restoring full binocular vision to amblyopic patients.
Accordingly, there is a great need in the art for an improved method of and apparatus for diagnosing and treating amblyoptic conditions within the human vision system, while avoiding the shortcomings and drawbacks of prior art apparatus and methodologies.